In many cases, hydrogen gas is used as a fuel gas for fuel cells. As such hydrogen gas, a hydrogen gas rich reforming gas which is obtained by reforming methanol or the like is used. When manufacturing such a reforming gas, a tiny amount of carbon monoxide (CO), namely several tens ppm to several hundred ppm, is present as impurities. For this reason, when such a reforming gas is used as a fuel gas for a fuel cell, the CO gas is adsorbed on the surface of the platinum catalyst of the fuel cell electrodes, thus hindering ionization of the hydrogen gas and lowering the output of the fuel cell. In order to take appropriate measures to counter such a problem caused by the CO gas, it is necessary to continuously monitor the concentration of CO gas in the reforming gas used in the fuel cell.
Conventionally, as for the most commonly used CO gas sensor, there are known a controlled potential analysis type CO gas sensor and a semiconductor type CO gas sensor. However, for the reasons given below, neither of these CO gas sensors is appropriate for detecting CO gas in a reforming gas.
Namely, the reforming gas contains hydrogen gas used as a fuel in the fuel cell for the amount of about 75% thereof. In comparison with this, the reforming gas contains a relatively tiny amount of CO gas as described above. Therefore, it becomes necessary to detect or measure CO gas in a hydrogen gas atmosphere containing a relatively large amount of hydrogen gas. However, in the case where the concentration of CO gas is measured in such a hydrogen gas rich atmosphere using these CO gas sensors, there is a problem that it is difficult to accurately detect (qualitative analysis) or measure (quantitative analysis) such CO gas with either type of CO gas sensor due to influence of the hydrogen gas rich atmosphere in which interference by hydrogen gas occurs.
In view of the problem mentioned above, it is an object of the present invention to provide a CO gas sensor which can accurately carry out detection (qualitative analysis) and measurement (quantitative analysis) of the concentration of CO gas when CO gas is detected or measured in a gaseous atmosphere containing a relatively large amount of hydrogen gas and carbon dioxide gas, a fuel cell power generating apparatus equipped with such a CO gas sensor, and a method of measuring the concentration of CO gas.